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1 Oceanological Studies

1 1996

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Oxygen and nutrients in the southern Baltic Sea

100%

Trzosinska A. , Lysiak-Pastuszak E.

nr 1-2

41-76

Temporal variations (1979-1993) in oxygen and nutrient distribution were inveastigated in the Polish sector of the Baltic Sea with reference to hydrological and biological factors.The most strinking differnce in oxygen condiotions in 1989-1993, as compared with the previous decade consist in much earlier and greater supersaturation of the euphotic zone and a significant reduction of oxygen deficiency in the deep water layers.The overall negative trens in oxygen concentrations observed at the bottom of the Gdansk Deep since the 1960s has been reversed.No hydrogen sulphide was found there from spring 1990 until late 1994.In 1989-1993 the winter accumulation peaks of all nutrients shifted from MaArch to February.InN the coastal areas both phosphate and silicate winterpools were reduced to their lowest levels already in spring; in the off-shore waters their respective minimal concentrations were reached in summer and autumn.Nitrates were used by May/June in all areas, except for the Vistula estuarya where their stock was sustained throughout the whole year.Distinct changes in the winter accumulation of nutrients in the surface water were found.In the Gdansk Deep the strong negative trend in silicates ceased, probably due to the declining demand for this nutrient.Rapid accumulation of nitrates was no longer in evidence, but the increasing N/P ratio could be traced in the Gulf of Gdansk as far as the Gdansk Deep.Moreover the coincidence of an effective phosphate sink and low denitrification activity has prompted a significant increase in the N/P ratio in the deep waters of the Gdansk Basin.The input of nutrients from rivers, with a considerablre surplus of nitrogen over phosphorus, is the most obvious reason for the advanced eutrophication in the Polish marine sea.Regional differences in the trophic levels are discussed on the basis of the N/P ratio in the water and the nitrogen and phosphorus uptake ratio.